Field of the Invention
The present invention relates to an image forming apparatus (e.g. a digital copying machine, a multifunctional peripheral, or a laser printer) for performing a correction method for correcting distortion and uneven image density during image formation of a two-dimensional image.
Description of the Related Art
In electrophotographic image forming apparatus such as a laser printer and a copying machine, there has been generally known a configuration to form a latent image on a photosensitive member through use of a light scanning device configured to perform scanning with a laser beam. In the light scanning device of a laser scanning type, a laser beam collimated through use of a collimator lens is deflected by a rotary polygon mirror, and the deflected laser beam is formed into an image on a photosensitive member through use of an elongated fθ lens. There is known multibeam scanning in which a laser light source having a plurality of light emission points is included in one package so as to perform scanning with a plurality of laser beams simultaneously.
In order to form a satisfactory image without uneven image density and banding, it is desired that distances between scanning lines of a laser beam scanning positions adjacent in a rotational direction of the photosensitive member be equal to each other. However, the distances between the scanning lines are varied due to a plurality of factors described below. The distances between the scanning lines on the photosensitive member are varied by, for example, a fluctuation in a surface speed of the photosensitive member, or a rotation speed fluctuation of a rotary polygon mirror. Further, the distances between the scanning lines are also varied by a variation in angle of mirror faces of the rotary polygon mirror with respect to a rotary shaft of the rotary polygon mirror and a variation in intervals between light emission points arranged on a laser light source. FIG. 18A is an illustration of a state in which an interval between the scanning lines is varied periodically, with scanning of laser beams being represented by horizontal lines. As illustrated in FIG. 18A, when the interval between the scanning lines of laser beams is small, an image is developed darkly. When the interval between the scanning lines of laser beams is large, an image is developed lightly. Thus, this development is liable to be detected as moire and the like. To cope with uneven image density and banding caused by such factors, there has been proposed a technology of correcting banding by controlling an exposure amount of the light scanning device. For example, in Japanese Patent Application Laid-Open No. 2012-098622, there is described a configuration in which a beam position detection unit configured to detect a beam position in a sub-scanning direction is arranged in the vicinity of the photosensitive member, and the exposure amount of the light scanning device is adjusted based on scanning distance information obtained from a detected beam position, to thereby make banding less noticeable.
In the image forming apparatus, when a printing speed is changed in accordance with the type of a recording material, thinning control, e.g., face skipping control is performed in some cases. For example, in the face skipping control, light exposure is performed once for a plurality of times of scanning. Through the thinning control, the printing speed can be changed without performing control of changing the speed of a motor of a rotary polygon mirror.
In the related-art method of adjusting density based on the exposure amount, an optimum amount of controlling a light amount is varied depending on a change in image forming conditions of the image forming apparatus. Therefore, it is difficult to perform banding correction stably. As the changes in image forming conditions, there are given, for example, a change in ambient temperature environment of the image forming apparatus, a change in sensitivity to light of the photosensitive member, and a change in a relationship between density and a light amount due to a change with time of characteristics of a toner material.
In a color image forming apparatus, when positional deviation occurs at a relatively long period, positional deviation occurs between colors at a long period to cause an image defect, e.g., uneven color quality. FIG. 18B is an illustration of a state of positional deviation of each scanning line. When printing is performed at a resolution of 1,200 dpi (scanning line interval: 21.16 μm) with respect to an image formed on a sheet having an image width of 297 mm in an A4 longitudinal direction (hereinafter simply referred to as “image”), about 14,000 scanning lines are formed. Due to the factors, e.g., a fluctuation in surface speed of the photosensitive member, the positional deviation amount between an ideal position of the scanning line and an actual scanning position in an image area is varied in a non-uniform manner. In FIG. 18B, in the 2,000th line and 7,000th line from a leading edge of an image, the scanning position of a scanning line represented by the solid line is deviated in a front direction from an ideal position represented by the broken line, and in the 10,000th line, the scanning position is deviated in a direction opposite to the front direction. When the scanning line, that is, the image position is deviated from the ideal position in the image area, a problem, e.g., a color quality variation occurs, and hence a configuration to move the absolute position of image data is required.
At a time of the face skipping control, there is a problem in that banding cannot be suppressed appropriately with a related-art optical face tangle error (banding) correction. In the related-art optical face tangle error correction, correction data is obtained from upper and lower regions in the sub-scanning direction of a target pixel to be corrected. When the correction data is obtained from the upper and lower regions in the sub-scanning direction of the target pixel, there exists a face that is not used for the face skipping control. With this, there is a problem in that proper banding correction may not be performed uniformly.
It is an object of the present invention to reduce an image defect, e.g., banding and color misregistration, even when thinning control is performed.